Thin film magnetic heads are primarily used in magnetic storage systems to write/read information in the form of magnetic pulses to/from a relatively moving magnetic medium. A magnetic transducer such as an inductive or magnetoresistive (i.e., MR) head is typically formed on a slider which is then mounted to a suspension arm of an actuator. The suspension arm suspends the head in close proximity to a disk surface.
Head supporting sliders are generally fabricated from a thin wafer of substrate including a matrix of thin film magnetic heads formed on one of the wafer surfaces. A number of known fabrication techniques may be used to form the heads, e.g. , sputtering, vapor deposition and plating. The particular processes used will depend on the type of head being constructed, but generally each process includes a stage wherein terminal pads or studs are formed at the slider's trailing edge for providing an electrical contact to the head element. For example, the formation of an inductive head can be separated into four stages: the construction of the first magnetic pole; formation of the conductor coil; construction of the second magnetic pole; and formation of the electrical conductors (studs). Four terminal pads or studs are usually required for each head. Upon completion of the studs, wafers are sliced and diced by known methods to form individual sliders. The sliders are then bonded to suspension arms, and leads from the suspension are soldered to the studs. Relatively large conductors are used in thin film heads to provide desired characteristics of mechanical stability, chemical stability and low contact resistivity.
According to previous methods, two different stud design types are commonly used in the fabrication of thin film heads. A first design comprises full gold studs. As the second pole tips of the magnetic heads are formed on a wafer, a layer of pole tip material is simultaneously applied at each stud location to form a first stud layer. Copper lead straps for coupling the magnetic head and terminal pads are applied next, forming a second stud layer. A NiFe seed layer is applied to the Cu layer of each stud, and a thick gold layer is plated to the NiFe using RISTON.RTM., a commercially available dry photoresist. The wafer is coated with a thick layer of alumina. The alumina "overcoat" is then lapped to expose the coated studs and to planarize the wafer surface.
A number of problems arise using the full gold stud design just described. For instance, use of a photoresist such as RISTON.RTM. causes capillaries to form along the thin film head structure because the resist is laminated rather than spun onto the wafer. Etching solution seeps into the capillaries, forming holes in the overcoat layer along the pole tip structure. Another problem occurs when the gold plating solution contains thallium as a grain refiner. The thallium aggregates at the NiFe/Au interface, weakening the bonding strength of the two metals. This condition leads to stud "pullout" when moderate thermal stress is applied to the exposed stud surface.
The second commonly used stud design comprises a thick Cu layer and a thin Au layer. As the second pole tip of the magnetic head is formed, a layer of pole tip material is applied at each stud location to form a first stud layer. Formation of Cu lead straps provides the next stud layer. A second, thick Cu layer is applied to each stud area using a spin-coated liquid photoresist. The wafer is then coated with a thick alumina overcoat which is lapped to planarize the wafer surface and to expose the copper studs. A thin gold layer is applied to each stud, again using a spun-on liquid photoresist. The gold layer studs are then lapped.
Although Cu/Au studs overcome some problems associated with full gold studs, they introduces other disadvantages. For example, more process steps are required to implement this design alternative. In addition, the number of metal interfaces and combinations are more complex. Application of each metal layer requires processing steps that increase risk of contamination (e.g., sputter deposition, photolithography, plating and etching). Contamination leads to degradation of bonding strength between layers.
Another problem arises from the use of liquid photoresists. Since the thickness or height of the thick second copper layer generally exceeds the height of the developed photoresist, the copper layer tends to "mushroom" at the point where the photoresist "wall" ends. The stud is therefore structurally weak at its base and may crack during lead bonding. When the resist is removed and the alumina overcoat is applied, the "neck" of the mushroom-shaped Cu structure remains uncoated. Such buried cavities can reduce head reliability if minor cracks in the alumina oxide appear and permit corrosive humidity to enter the space.
It is also possible for the Cu to be exposed even after gold plating, resulting in corrosion in the presence of humidity. The thick copper layer requires a copper seed layer as a plating base. The seed layer must be removed after plating and before application of the NiFe seed layer by using a sputter etch technique. Sputter etching, however, causes some of the copper to redeposit at the second pole tip structure, leading to corrosion at the resulting Cu/NiFe interface.
In either stud design described, a thick alumina overcoat is required to overcome topology differences between the thin film head and the studs. Deposition of a thick overcoat may have a duration of more than 16 hours since the deposition rate of alumina is low. Moreover heating temperatures during deposition may reach temperatures exceeding 120.degree. C. Consequently, the wafers are thermally stressed, resulting in a degradation of overall magnetic behavior, particularly in the production of MR heads.
What is needed is a process for forming electrical conductors that overcomes the problems of contamination, overcoat damage, stud adhesion failure, corrosion and degradation of magnetic properties without substantially adding to the complexity or cost of head production.